1. Field of the Invention
The present invention relates to an image processing method and an ink jet printing apparatus. More specifically, the present invention relates to an image processing method and an ink jet printing apparatus for the case of performing maintenance ejection onto a printing medium for the purpose of maintenance of an ink nozzle for ejecting ink.
2. Description of the Related Art
An ink jet printing apparatus has various advantages including low noises, low running costs, capabilities of downsizing the apparatus and color printing, and so forth. Such an ink jet printing apparatus is configured to print an image on a printing medium by ejecting ink from ink ejection nozzles provided in an ink jet printing head. The trend is that ink droplets ejected therefrom are becoming smaller from about 15 pl to 5 pl, or further to 2 pl. Thus, graininess is reduced, for example, in a halftone part of a gray-scale image, in a gradation part of a color image and in a highlighted part. Along with reduction in size of droplets, an aperture size of an ink ejection nozzle orifice is also reduced.
Incidentally, water evaporation of the ink in the vicinity of the nozzle orifice is accelerated when the ink is not ejected from the ink ejection nozzle. A clogged orifice caused by an increase in viscosity of the ink leads to defective ejection such as deviation in the direction of ink ejection and failure of an ink droplet to reach a sheet surface. Moreover, such defective ejection tends to occur more often when the nozzle orifice has a small orifice diameter. Meanwhile, the defective ejection may also occur due to other reasons such as adhesion of minute paper powder or water droplets on the nozzle orifice.
To solve aforementioned problems of the defective ejection, an ink jet printing apparatus has been conventionally configured to perform ink ejection outside a printing sheet area immediately before printing or at periodic intervals in the course of printing. This maintenance ejection which is ink ejection aiming at maintenance is performed either arbitrarily or periodically at a waste ink absorber, an aperture region exclusively used for maintenance ejection and the like, which are disposed inside the printing apparatus. Although a scale of maintenance ejection varies depending on ejection power of a printing head used therein, a drying performance of ink used therein, an environmental temperature, and the like, several shots to over a dozen shots of ejection are usually performed at an interval of about 3 to 10 seconds.
When this maintenance ejection is performed during a process of printing an image on a printing medium by scanning the medium with a carriage mounting the printing head, the carriage has to travel from a position for printing the image to a predetermined position outside the printing medium where the waste ink absorber is located. Therefore, required time duration from start to end of printing the image on the printing medium is extended. In particular, in a high-speed printing mode for minimizing a printing operation per page by use of high-speed printing head movement and maximum performance ejection of the printing head for the purpose of high-speed printing, a loss attributable to the maintenance ejection may occupy a considerable portion from several to over ten percent relative to the total time for printing the image.
Now, a case of performing printing based on data equivalent to one line of a band width of a printing head length on a page by one scanning of the printing head will be described below as an example. A printable area of an A4 printing sheet is defined as being 11 inches long and 8 inches wide, and an image will be printed using a 0.53-inch printing head provided with 320 nozzles for accommodating an ink droplet amount of 30 pl each and a pitch equivalent to 600 dpi. The printing head has to scan 21 scans (movements of the printing head+line feeds) to print an image over one page. Moreover, when a drive frequency of the printing head is equal to 15 kHz and dot density in the scanning direction is equal to 600 dpi, a scanning speed of the printing head is equal to 25 inches per second. Estimating that line feed time, rise time or fall time of the movement of the printing head (rank up and rank down) is equal to 0.1 second each, printing duration per line is approximately equal to 0.52 second. Therefore, a total time duration required for printing on one A4 paper is approximately equal to 11 seconds. Further, estimating that an interval of maintenance ejection is set to 5 seconds, the maintenance ejection will be performed twice during printing on one page. One scan row is inserted for one maintenance ejection operation separately from the printing operation. Accordingly, a rate of the time used for the maintenance ejection to the time used for printing the image is calculated as:
                              2          ⁢                                          ⁢          scans          ⁢                                          ⁢          for          ⁢                                          ⁢          maintenance          ⁢                                          ⁢                      ejection            /            21                    ⁢                                          ⁢          scans          ⁢                                          ⁢          for          ⁢                                          ⁢          printing                =        0.092                                =                  about          ⁢                                          ⁢          10          ⁢          %                    
On the other hand, another conventionally known technique for maintenance ejection control to reduce the time loss associated with the maintenance operation is configured to eject ink on a printing medium not for the purpose of image printing. According to technique, movement of a carriage associated with a maintenance operation is reduced to decrease a time loss. Japanese Patent Application Laid-open No. 8-112904 (1996) discloses a technique for maintenance ejection control focused on which region (such as a white portion without an image, a black image portion and a colored image portion) of a printed image maintenance ejection should take place. Specifically, this publication discloses a method of ejecting ink in the vicinity of an edge of an image or a method of forming a watermark on a printing medium so that such a visible image does not cause a problem.
However, the place, the number, and other parameters of maintenance ejection onto a printed image are preset in the conventional example described above, and the maintenance ejection is performed regardless of presence of an image to be printed on a printing medium, ink colors, and the like. In this concern, the pattern of the maintenance ejections may be visible as an image depending on the presence or absence of a printed image on a printing medium on which the ink is ejected by the maintenance ejection, and a relation between the color of ink to be ejected by the maintenance ejection and the color of the printed image.
In other words, visibility of ink dots as a result of the maintenance ejection tends to be different between the case of performing the maintenance ejection on to a white portion of a printing medium and the case of performing the maintenance ejection on a printed portion. Such a difference may pose visibility problems of the ink dots in some cases.
For example, when cyan ink is ejected as the maintenance ejection onto a yellow solid image, such ink dots are prone to be more visible (conspicuous) than ink dots ejected onto a white region.
FIG. 1A and FIG. 1B are views schematically showing print samples subjected to visibility judgments of ink dots ejected as the maintenance ejection onto printing media such as paper sheets. FIG. 1A shows a state of performing the maintenance ejection of one dot onto a white portion of the paper sheet while FIG. 1B shows a state of performing the maintenance ejection of one dot onto a solid portion in a printed image in a different color from that of the image. Here, a lattice 210 is provided for schematically illustrating the print resolution and is not printed in actuality.
FIG. 1A shows the ink dot formed by the maintenance ejection onto the white portion and FIG. 1B shows the ink dot formed by the maintenance ejection when there is a solid image in a different color as a background. When there is the solid image in the different color as a background, an ink dot of the maintenance ejection bleeds into solid image ink dots 220 of the different color and the shape of the ink dot spreads irregularly (reference numeral 230). Accordingly, visibility of the ink dot ejected as the maintenance ejection is increased.